drivers/include/sx127x.h: fix power var len

drivers/include/sx127x.h

@@ -208,7 +208,7 @@ enum {
  */
 typedef struct {
     uint16_t preamble_len;             /**< Length of preamble header */
-    uint8_t power;                     /**< Signal power */
+    int8_t power;                      /**< Signal power */
     uint8_t bandwidth;                 /**< Signal bandwidth */
     uint8_t datarate;                  /**< Spreading factor rate, e.g datarate */
     uint8_t coderate;                  /**< Error coding rate */
@@ -616,7 +616,7 @@ uint8_t sx127x_get_tx_power(const sx127x_t *dev);
  * @param[in] dev                      The sx127x device descriptor
  * @param[in] power                    The TX power
  */
-void sx127x_set_tx_power(sx127x_t *dev, uint8_t power);
+void sx127x_set_tx_power(sx127x_t *dev, int8_t power);
 
 /**
  * @brief   Gets the SX127X preamble length